1. Field of the Invention
The present invention concerns an arrangement for activation of a thermotransfer print head as well as a method for activation of a thermotransfer print head. The invention is particularly suited for use in franking machines, address machines and similar accounting or mail processing apparatuses.
2. Description of the Prior Art
The thermotransfer franking machine T1000 manufactured by Francotyp-Postalia has a thermotransfer print head, mounted fixed in a housing, for printing a franking imprint and a tray, externally attached to the housing, to accept an exchangeable thermotransfer ink ribbon cartridge. The tray encloses a non-secure region. A mail piece is moved through the printing station synchronized with the thermotransfer ink ribbon, the movement being monitored by a detector that generates an output signal representing a parameter proportional to the ribbon movement (European Application 18 92 69 equivalent to U.S. Pat. No. 4,705,417).
Although a door leading to the tray can be opened at any time, access to the secure region of the printing device is prevented by a security housing. Due to the security housing, no special security measures must be taken to protect the activation and data signals for the print head that allows a printing of fixed, semi-permanent and variable information (U.S. Pat. No. 4,746,234).
For the thermotransfer print head, it is known from German OS 05 38 33 746 to integrate an internal switching unit, charged via an external activation unit, into the print head that contains the thermo-printing heating elements in a single row, which enables a selective activation with pre-heating of the thermo-printing heating elements to reduce the heat output upon printing. The resistance heating elements are directly pre-heated to a pre-heating temperature with a clock frequency adapted (in terms of pulse amplitude and pulse width) to the necessary heat energy. At the end of the printing time, the pre-heating temperature is maintained with such a clock frequency.
A method for control of the feed of a thermo-printing heating element is disclosed in European patent 536 526 B1. A print requirement is determined in advance at the respective raster points in time of a predetermined print raster. An output of current pulses to the respective thermo-printing heating elements ensues both for the raster points in time without a print requirement and for the raster points in time with a print requirement. The current pulses (pre-heating pulses) (which are output according to a specific algorithm before a raster point in time with a print requirement) effect a pre-heating of the respective thermo-printing heating element up to a temperature just below a limit temperature at which a print point is delivered by a thermotransfer ink ribbon and is visible on a carrier material (mail piece). Pre-heating pulses cannot be output in a sequence that is too fast nor at intervals that are too large for the respective thermo-printing heating element, because otherwise the aforementioned limit temperature would be exceeded or undershot. In the first case, the print image appears too heavy and smeared. In the second case the print image is too thin and pale because by itself the main printing pulse effects only a short-term exceeding of the limit temperature at the raster point in time with a print requirement.
A method is also known wherein a predetermined pre-heating temperature is maintained in the printing pauses at the respective heating element by means of pre-heating and post-heating pulses (German OS 38 33 746).
A controller that, dependent on the print head temperature, influences the pulse width or amplitude of the heating pulses in order to achieve overheating protection is known from U.S. Pat. No. 4,510,507 and German OS 33 27 904.
A print head thermo-controller is disclosed in European Patent 730 972 wherein the power electronic associated with the print control unit regulates the amplitude of the print head voltage, corresponding to the environment temperature and is combined with a control unit that operates according to an anticipatory control algorithm for feeding individual thermo-printing heating elements with pre-heating pulses and printing pulses of variable pulse duration.
For such a franking machine, a method and arrangement for fast generation of a security imprint are disclosed in the European Patent EP 576 113. The method enables embedding of variable data during the printing of the security imprint, but this then allows only a brief projection to determine a print requirement.
Very high requirements are placed on a security imprint by some postal authorities, in particular with regard to its machine-readability and communications about auxiliary services of the postal carrier that can change from letter to letter. Since April 2004, Deutsche Post AG has promoted the launch of the first franking machines in Germany with a digital indicium “FRANKIT”: (/www.deutschepost.de/download/broschueren/20403000_Frankit_Folder.pdf). The following are encrypted in a matrix code:                a) all specifications readable in plain text, such as date, postage value etc.,        b) information regarding the franking type, product code, current shipment number, machine identification and serial number,        c) copy protection information.        
Such a security imprint contains previously entered and stored postal information including the postal rate data for transport of the letter and, if applicable, a marking with security information. In modern franking machines, the accounting and storage of postal rate data (European Application 789 333) and internal security measures (U.S. Pat. No. 6,351,220, and German Utility Models 299 05 219, and 201 12 350) are implemented and the aforementioned security information are generated (German OS 199 28 058, U.S. Pat. No. 6,041,704) by a postal security module arranged inside the meter housing. The prior calculation of security information requires a majority of the time in the postal security module, and thus the security information is available for embedding into the print image only relatively late. Even a partial prior calculation of security information well before a franking by the franking machine can not prevent that the matrix code in the marking field from changing from mail piece to mail piece. This makes it more difficult to still determine a print requirement in advance in a timely manner. The printout of a machine-readable matrix code requires a higher number of raster points in time, corresponding to the higher print resolution, which is associated with a higher computing capacity. A requirement for a 25-50% faster mail piece transport also has a detrimental effect. The raster points in time follow one another in shorter intervals the higher the selected mail piece transport speed. If the thermo-printing heating elements are pre-heated by means of pre-heating pulses up to a preheating temperature up to relatively close to the aforementioned limit temperature without exceeding the latter, the maximum possible duration of the pre-heating pulses is limited by the reduced intervals between the successive main heating pulses. The (in practice controllable) maximum possible pre-heating pulse height is likewise limited. Conventional methods for thermotransfer printing control the temperature at the individual thermo-printing heating elements of the print head via the most varied methods. Given a high print image resolution and transport speed, the print image of the first print columns appears to be printed more faintly at the beginning of the printing than in the remaining print columns of a stamp imprint. Moreover, a wave-shaped repeating attenuation in the print pattern (Ratter effect) acts in an interfering manner in the remaining print columns the higher and more non-uniform the mail piece transport speed is during the printing. If a transport delay occurs for any reason, the resistance heating elements that generate the print image points (dots) cool, and given further printing a section of the print pattern is printed that appears somewhat fainter since the temperature is no longer reached. This can only then be adjusted again after more than one further print column has been printed.